The performance of a steam turbine is primarily determined by design of the steam path components, including the nozzles and bucket shapes. In that steam turbine path, the kinetic energy in the steam is converted to rotational energy in the turbine shaft, for example, for driving a generator. More particularly, the pressure and temperature of steam (and therefore the enthalpy) at any given intrastage location represents potential energy. The nozzle portion of a stage converts some of this potential energy into kinetic energy. The buckets following this nozzle portion convert this kinetic energy into rotational or shaft energy. This process repeats for each successive stage until the potential energy prescribed to remain at the exhaust of the stage group is reached. It is therefore important to minimize aerodynamic losses in the steam path to improve efficiency, while maintaining reliability and cost effectiveness.
Conventional practice in the design of a turbine stage is to provide for radial equilibrium along the flowpath between the partitions and end walls such that constant axial velocity and work is achieved at all radial positions or radial heights of the nozzle. Free vortex designs such as this have long been utilized, with the throat areas between partitions increasing linearly with radial outward distance from the hub of the stage. In certain applications, however, controlled vortex or reverse designs have been used in which the flow is biased radially inwardly in efforts to minimize secondary flow and profile losses in the hub region of the bucket receiving the nozzle flow. Impulse stage design results in bucket hub regions with large amounts of turning and historically this area has a larger fraction of the overall loss. Energizing this region with the reverse aerodynamic approach can have a beneficial result. However, this needs to be accomplished without causing hub and tip nozzle vortex interaction. If interaction does happen, this can substantially reduce the benefit of the reverse aerodynamic design. Generally speaking, a reverse design provides decreasing throat areas between partitions with increasing radial height. Significant secondary aerodynamic flow losses, however, can be caused by secondary flows (vortices) generated as the boundary layers along the inner and outer side walls of the turbine nozzle rows are turned through the nozzle. The stronger the vortices, the greater the losses. In the course of investigating the reverse design, it has been found that the vortices forming the secondary flows along the tips and bases of the partitions (adjacent the hub and tip end walls) can interact with one another to increase the aerodynamic losses. There appear to be cases when the throat passage geometry in particular effects the conditions for interaction. In addition, the reverse design may decrease the nozzle flow adjacent to the outer side wall too much, resulting in a poor flow distribution into the bucket, and excess swirl near the tip at the stage one exit plane. Hence, further refinements are necessary to the controlled vortex design to optimize radial flow distribution and minimize aerodynamic losses.